We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

Everything from "so lets add some constraints" is really an answer (albeit an incomplete one), not part of the question. You should edit that part out (and perhaps add a little bit of an explanation of why it's worth knowing this) and add it as an answer that someone can build on.
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Tony MeyerNov 12 '11 at 3:33

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This question seems to fall into a grey area of sorts. Technically it's valid, but then so would all questions of the form "What is the <attribute> of <character> in <show>?" Unless it is somehow relevant to the plot, that is. Also, is My Little Pony technically fantasy?
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HNLDec 14 '11 at 10:21

11 Answers
11

A: The ponies' ages were left intentionally undefined. I like to think of them as having maturity levels anywhere between 12 and 18, (but remember that maturity is different than age. You can be a 10-year-old with the maturity of a 15-year-old, and you can be a 35-year-old with the maturity of a 15-year-old.) For story telling purposes, they needed to be able to live independent of parents, but innocent enough for the type of lessons they learn at the end of the episodes. Since horses are full grown around 5-ish, I guess I sort of justified it in my head that they could be childlike young adults. I know this answer is frustrating for some people, but it happens often in cartoons. How old is Bugs Bunny? Mickey Mouse? Sponge Bob? I don't know either.

So it's undefined, but it's strongly implied that they are 3-5 years old in pony years. I highly doubt this is their actual age, and I suspect MLP ponies age differently to RL ponies.

Edit: I've just checked wiki and the oldest age for a pony horse is 30 years, though that is only the average. Some unusual horses live to be 40, with the oldest pony at 57. Let us assume that Granny smith is about 50. She's old but not so old as to be dying all too soon. If we make her at about 200 years old in MLP years (due to her being there at the start of ponyville which is centuries old), that makes an age contraction of about 1:4. Then checking back to Wiki, any horse pony older than 4 years is a Mare or a Stallion. I'd then place their ages between 16 and 20. We've got to bare in mind this though:

Regardless of a horse's actual birth date, for most competition purposes, an animal is considered a year older on January 1 of each year in the Northern Hemisphere and August 1 in the Southern Hemisphere. The exception is in endurance riding, where the minimum age to compete is based on the animal's actual calendar age.

I think we're safe in the assumption that we're in the northern hemisphere. From this all the ponies ages are 'the same', except Applejack who is probably considered a year older due to using her actual date of birth, I'd make her ~5 but only just and living with such young friends makes for her not acting her age. I'd probably sum all of this up to say they're all about 17-18 in MLP years (4-5 in RL pony years). In the episode "Griffon the Brush-Off" Fluttershy retorts to a line said by Pinkie Pie by saying "I'm a year older than you", which could either be interpreted in pony years or our years, giving the age of either 5 pony years or something like 19 or 20 of our years. Another thing is how Pinkie is actually far smarter than she initially seems, implying much more maturity (which again implies age) than at first glance.

That makes our best guess something like this (to the nearest year) in RL/MLP terms:

Twilight Sparkle - 4/18 (fresh out of magic school)

Pinky Pie - 4/18

Rarity - 4/18

Apple Jack - 5/20

Rainbow Dash - 4/19 (rounding down to 4 due to the January birth thing)

Granny Smith is at least a couple of hundred years old. This is shown in Family Appreciation Day as she is one of the first to settle in Ponyville and in Winter Wrap Up, Twilight Sparkle says that Ponyville's been wrapping up winter for hundreds of years without magic. Just put two and two together and you can tell Granny Smith must be at least a hundred years old.

I don't know a lot about pony aging, but I'm pretty sure I have a solid ageing system for the cast in human years - translate them to pony years if you wish - but then the Fluttershy = Pinkie Pie + 1 rule jams it up a bit.

RULES AND OBSERVATIONS:

Assuming that pony school, pegasi school, and magic school start at
the same time.

Fluttershy is older than Rainbow Dash.

They are all considered adults.

I'm using flashbacks as references.

I'm using height and sizes.

Also using season 1 episode 23 flashbacks.

That all the schools begin enrolling at the age of six.

Fluttershy is the largest, tallest, longlegged, wide wingspanned, and
is already enrolled in school (enrolled two years prior to Twilight,
one year above everyone else).

Rainbow Dash is also enrolled in the same school but is younger than
Fluttershy.

Rarity is already enrolled in a school, and is allowed access to
sharp objects for making costumes.

Apple Jack is old enough that her guardians let her travel alone to a
big city, though is similar in size to Rarity and Rainbow Dash
(since they don't
show Apple Jack attending or enrolling school and I'm using school and size as a guide, my guess for her is
uncertain).

Pinky Pie never attended school. She presumably went from working at a rock farm to
working in a bakery shop. She is the same size as Rainbow Dash, Apple Jack and Rarity.

Twilight is barely enrolled for school, but she is the same size as all but Fluttershy.

AGES:

Fluttershy is the oldest at the age of 20 (one year older than Pinky and
enrolled two years prior to RD so she's much larger).

Apple Jack and Pinky Pie are 19 (AJ and PP never attended school, at
least as shown, but AJ does mention getting her cutie mark last).

Rainbow Dash and Rarity are 18 (RD and Rarity both were enrolled at the same year. That's why they are the same size).

Twilight Sparkle is the youngest at 17 years old (She is Princess Celestia's protege, but she was sent to Ponyville to CONTINUE her studies, therefore she hasn't graduated yet. She is also scared of being held back/sent back grades, inplying that she is still in school).

That said, we don't see the reverse side of the cake (where there could be more candles), and the celebration is Rainbow's "Birthiversary" -- a simultaneous celebration of her birthday and the anniversary of her arrival in Ponyville, so the candle count could be a combination of the two.

I think the most reasonable explanation would be to say that regular ponies age at the same rate as humans. For example, a six year old pony would be the same as a human child. I base this off Crystalis' comment to Pinkie Pie in A Canterlot Wedding.

So, the mane six are in their late teens, although they don't always act like it. Alicorns are either immortal or have very long lives. Princess Luna is physiologically not much older than the mane six, and about the same as Princess Cadence. Speaking of who, Cadence doesn't seem to be as powerful as Luna and Celestia, so she may age like regular ponies even though she's an alicorn. It may explain why she chose to marry and why Luna and Celestia never have.

As for Granny Smith, let's look back at our own history these last hundred years. Even as cars and electric lights were being created, there were still Civil War veterans alive. So I'd say Granny Smith is at least 90 years old, and that the age of Ponyville was exaggerated.

Oh ok. I think my ages were SLIGHTly irrational. Pinkie lives and works for strangers?! That invalidates my guess for Pinkie. So I would start a baseline age for... Let's say 15. The only thing, is that she is really hyper and weird (ADHD, i believe) which scales her age down and the reason why I put her as 10. So I'll upscale the ages of the mane 6... (Multiply by 1.5, rounded down) Also Fluttershy is 1 pony year older, not 2.2.

As I said, Pinkie Pie is 15. By the end of season 2, she must be 19 to 21.

Twilight Sparkle is 18. This is valid, because she is fresh out of magic school. By the end of season 2, she must be 22 to 24.

Rainbow Dash is 16. By the end of season 2, she must be 20 to 22.

Apple Jack is 22.

From the premises, Flutter Shy is 16 as well. This is valid, as they were friends for a long time. By the end of season 2, she must be 20 to 22.

Rarity is 24.

Celestia and Luna must have been existing from the Creation (I am Christian) MUST certainly be at least 2200 and may be at least 13200 (Creation happened around 4000BC) and Celestia is older than Luna.

Spike is 13 (Correction, 18-5). Twilight got him when she was five.
Granny Smith's age would STILL be 453-897 (1 human year is 2.2 pony years and 2-4 centuries would be reasonable. Also, she may have been 13 to 17 before Equestria was started.) Then again, that's pretty old.

Shining Armor should be 22-23 in human years (captain in the royal guard, which is comparative to the Marines or so)

I think they would have to be at least 17/18 due to the fact that they live alone and work..

Applejack will probably be the oldest because she said she got her cutie Mark last in her class ect. But earth pony school, magic school and flight school might have different starting ages and/or different grades so its kinda hard to judge.

I reckon they are about 19-21 with AJ and Rarity being the oldest and Rainbow and Fluttershy being the younger of the 6.

Im certain Pinkie is much older than she seems due to the fact that she lives with and works for strangers(as in not family) and is allowed to foalsit their young foals by herself.

I think enough has been said about Granny Smith, Luna and Celestia so Ill leave it at that. ^ - ^

We got some new information from this week's episode, S5 E11 Party Pooped. In this episode, Applejack finds some notes about future parties Pinkie Pie is planning.

Looky here. It's notes for the party she wants to throw for her folks' fiftieth anniversary. But they ain't nearly that old!

That probably refers to the wedding anniversary of Pinkie Pie's mother and father. Their wedding was probably before Pinkie Pie was born, or not much after, so this means Pinkie Pie and Fluttershy are much younger than 50 years old. (Alternately, this could refer to some other family member, or the Cakes, who treat Pinkie Pie as familiy.)

Now some information from earlier episodes. In S3 E8 Apple Family Reunion, Granny Smith says they've been holding reunions every hundred month since Ponyville was founded, and Auntie Applesauce mentions there has been at least six reunions before this one, which makes Ponyville at least 56 years old.

I assume. That what is being said in all the comments, they would be
17-20 years old. Since they are living home alone. Besides Pinkie Pie.

Applejack, Being the middle child of the Apple family, and a
worker for the family, and of course being the last to get her
cutie mark in her class, I would put her the oldest or second
oldest of the Mane 6.

To why Granny Smith looks old and wrinkly like she does, Luna and her sister are
Alicorns, and are immortal. They cannot die from age. So to add
common sense to add to this concept, if somepony is an immortal. You
would reach your max height, and looks, and stay that way. (AKA: Looks young forever)

For the ages of Fluttershy and Rainbow Dash, well, you could say they
may be the youngest of the other 6 ponies, but to me, I put them in
the middle. Let’s start with this though before going to Rainbow Dash
and Fluttershy ages, I myself could put Pinkie Pie around... 18 or 17 years of age- She
foalsits and she bakes at Sugar Cube Corner, and lives with strangers
she trusts. Would you trust a kid to be left alone at your house to cook and
baby sit your kids? I would not.

And now to Fluttershy. Fluttershy did say in an episode of My Little Pony she is a year older then Pinkie Pine. And as I said. Pinkie Pie is 17 or 18, so that means she is 18 or 19 years old.

For Rainbow Dash? I coooould say she is the same age as Pinkie Pie or Fluttershy, so 17-19 years old.

Reason for putting Fluttershy and Dashie close of age is,since
Dasihe and Fluttershy known each other since they were young and in flight school.

Now to Rarity. She owns her own tailor shop and makes her own clothes. So she can be around 20s? She could be hitching the same road with Applejack in the 20’s. To exactly tell who is the oldest between the two? Or if they are the same age. Not sure, but I would put them early 20's.

For Twilight Sparkle? I could put her between the ages of... 16-19.
Because Twilight being out of school in a manners of sorts, and being out of
kindergarten school at that as well.

And being the baby sister of Shining Armor, who Shining may be
around 30’s, due to him being the captain of the Royal Guard.

And to add more common sense to this whole statement, since she is
getting private schooling from Celestia, I would assume she may be
the youngest of the mane 6, placing her at the age of 16.

Being an animal and aging differently than humans, they're very young. If I was being a realist, a horse's life expectancy is only 20 to 30 years. A pony that is a couple months old may be equal to a human child, and a year old pony would be equal to a teen and so on.

I disagree with the other comments saying they are 15 or 17 because they would be old. The only ponies that look young forever are the unicorn gods. Time in that realm must be different, probably a human month would be like several months to them, or even a year.
This logic is based off of a horse's actual life span.

If this is not the case and their ages are equivalent to human years and time, then their ages are probably like 12 to 14 and at the most, 16. But it's hard to tell because everyone in the show has the personality of a little kid, same with Spongebob.